Field of the Invention
The present invention relates to updating of gamma correction information used to perform density correction for input image data in an image forming apparatus.
Description of the Related Art
Printing image data for which processing has been performed in an image processing apparatus, such as a computer, is widely performed. In general, an image processing apparatus performs density correction in accordance with gamma characteristics (input/output characteristics) for input image data and forms an image on a printing medium, such as paper, by using the image data after density correction. The density correction at this time is performed based on gamma correction information set in advance. Gamma correction information is set in advance, but the gamma characteristics change due to deterioration with the passage of time of a photoconductor drum, a transfer belt, etc., or exchange of parts. The gamma characteristics also change due to environmental fluctuations in temperature/humidity or the like. In the case where the gamma characteristics have changed, a prospected density cannot be reproduced unless the gamma correction information is updated in accordance with the change and there is a possibility that image quality deteriorates.
As a method of updating gamma correction information, for example, there is a method of updating gamma correction information based on density detection results by outputting a plurality of patch images corresponding to a plurality of tone level values onto a transfer belt or paper and by detecting densities of output patch images with a sensor or the like.
However, in the case where large environmental fluctuations have occurred from the normal state, it is no longer possible for an image processing apparatus to stably output a patch image and it is frequent that the output density of a patch image is higher than the density supposed from the amount of environmental fluctuations. For example, there is a case where the same density is detected from patch images at a plurality of tone levels. There is a case where the gamma correction information updated based on the density detection results such as those may cause a tone jump in which the gamma characteristics change rapidly.
In order to solve this problem, Japanese Patent Laid-Open No. 2008-085564 has proposed a method of selecting one of the highest density and the lowest density in each area in the case where the same density is detected in a plurality of patch images in the area in which density is low and a plurality of patch images in the area in which density is high. Due to the method such as this, the gamma correction information is updated so as to reduce the occurrence of tone jump.
However, in the method described in Japanese Patent Laid-Open No. 2008-085564, a plurality of patch images is output in the area in which density is low and in the area in which density is high, but in the case where patch images having the same density exist, the number of pieces of density information effective for setting a gamma correction curve is reduced. That is, a wasteful patch image is output in order to acquire density information necessary for setting a gamma correction curve, and therefore, there is such a problem that toner is consumed wastefully. On the other hand, in the method described in Japanese Patent Laid-Open No. 2008-085564, in the case where the number of patch images is reduced in order to avoid a wasteful output of a patch image, there is a possibility that it is no longer possible to correct density with a high accuracy in the area in which density is low and the area in which density is high.